The invention pertains to a screw with a shank provided with a thread along a part of its length, and a screw head, where between a thread located at the free end of the shank and the screw head there is at least one unthreaded shank section. Screws of this type are used, for example, in the mounting of sandwich elements to a wall and roof lining, in the attachment of roof elements like, e.g., corrugated plates, or even in the attachment of cover sheets to a flat roof. The screw grips into a solid substrate, e.g., a steel or wooden support or a sheet metal structure by means of the thread formed on the free end region, and projects relatively far away from this solid substrate. This occurs because the screws have to be appropriately long due to the shaping of the plates, the thickness of the sandwich elements and/or the thickness of the insulation to be penetrated.
Now there are always problems with these screws since they are subject not only to longitudinal and transverse tensile forces during this type of attachment, but also due to the bending. This bending load results from the differing temperature deformation of the solid under-structure and the external surface of the reinforced part. In a sandwich plate for example, a relatively high temperature difference is obtained between the outer and the inner shell when the outer shell is heated by solar radiation.
The fasteners commonly in use today in these applications are placed under severe stress due to this effect, since the entire bending of the screw head is transferred to the thread formed on the free end of the shank. This alternating bending in the region of the thread leads to crack formation in the threaded core after a corresponding number of alternating motions, and thus necessarily to its fracture.